Annotated classification

The classification presented here follows that of U.S. paleontologist George Gaylord Simpson, which is generally accepted but modified in some fairly minor ways by other authors. The term Mesaxonia, introduced by the 19th-century paleontologist O.C. Marsh, is essentially synonymous with Perissodactyla. Simpson used it as a convenient designation for the superorder containing the single order Perissodactyla, as he used the parallel name Paraxonia for the superorder with the one order Artiodactyla.

Groups indicated by a dagger (†) are known only as fossils.

Order Perissodactyla

Herbivorous ungulates with either 3 digits or 1, at least in the hindfeet. Early forms digitigrade, this feature either retained or completely replaced by the unguligrade condition in later representatives. The body weight borne mainly or entirely on the third digit through which the long axis of the limb passes (mesaxonic condition). The talus (heel bone) with only 1, proximal, keeled surface, articulating with the tibia, no duplication of keel on the distal surface, as in artiodactyls. Nasals broad at the posterior end, alisphenoid canal present. Posterior premolars molariform; cheek teeth bunodont in early forms, typically lophodont or selenolophodont, rectangular; wide diastema separates them from incisors and canines, which may be reduced or absent. Testes inguinal, occasionally scrotal; mammary glands inguinal, with 2 teats; uterus bicornuate. Fifteen living species, but more than 200 fossil forms.

Dentition complete, upper molars with 6 tubercles, the 2 external ones united to form an ectoloph, median and internal tubercles generally fused into a single loph. Tendency to molarization of premolars and reduction of lateral digits.

†Family Palaeotheriidae (paleotheres or “native horses”)

Lower Eocene to lower Oligocene; Europe and Asia. Size variable, from that of fox (shoulder height about 40 centimetres or 16 inches) to that of a rhinoceros; body form tapirlike. Three digits in each foot. Dentition complete or first premolars absent; premolars molariform, separated from canines by diastema. Cheek teeth brachydont, lophodont, with or without cement. Orbits without postorbital bar. Eurasian “native horses,” 7 genera.

Family Equidae (horses and relatives)

A progressive group with tendency to molarization of premolars and development of cement.

†Subfamily Hyracotheriinae (hyracotheres)

Eocene: Europe and North America. Size about that of a fox; 4 toes in forefoot, 3 in hindfoot. Dentition complete, premolars simple or last 2 slightly molariform, molars brachydont, bunodont, diastema short. Skull short, without postorbital bar. Three genera. Basic stock from which other members of family evolved.

Lower Miocene to Pleistocene of North America; Pleistocene of South America; lower Pliocene to Recent of Europe, Asia, and Africa. Most forms as large as ponies or modern horses. Feet with 3 toes in early forms, 1 in later forms (but metapodia 2 and 4 persist as remnants), limbs with progressively elongated lower segments and reduced or fused ulna and fibula. Calcaneum without articulating surface for fibula. First premolars reduced or absent, others molariform; cheek teeth hypsodont, selenolophodont, with cement; canines in long diastema. Skull long, with postorbital bar in upper Miocene and later forms; nasals long and narrow. Some Holocene forms with striped coat.

†Superfamily Brontotherioidea

†Family Brontotheriidae (titanotheres)

Lower Eocene to lower Oligocene. Fossils from North America and Asia. Early forms as large as tapirs, end forms huge, standing 2.5 metres (8.2 feet) at the shoulder. Graviportal browsers; 4 toes in forefoot, 3 in hindfoot. Dentition complete, premolars small and simple in early forms; later forms with incisors and first premolars reduced or absent, remaining premolars only partly molarized. Molars large, bunolophodont, above with W-shaped ectoloph as in early equids, lower with double V resembling equid pattern but lacking reduplication of cusp at union of V’s. Skull elongated, low, without postorbital bar, with large rough, hornlike process in later forms. About 39 genera.

Upper Eocene to lower Pliocene. Fossils from North America, Europe, Asia. Early forms equivalent in size (and similar) to contemporary horses,
Hyracotherium; later representatives larger, up to size of a modern horse. Most with forelegs longer than hindlegs and 3 toes in each foot; rudimentary metacarpal V retained in forefoot, most with clawlike digits. Incisors of canines reduced or absent; first premolar absent, remainder small and simple. Molars bunolophodont, cusp pattern similar to Brontotheriidae. About 13 genera. Aberrant perissodactyls which may have used claws to dig for bulbs and roots.

Lower Eocene to Holocene. Fossils from North and South America, Europe, Asia, Africa. Recent species in Central and South America and Southeast Asia. Limited to tropical equatorial forests of South America and Asia. Browsers. Skull greatly modified; premolars completely molarized, first sometimes absent; transverse lophs strongly developed in molariform teeth, ectoloph simple. Limb structure primitive; 4 toes in forefoot; 3 in hindfoot. Six genera but only 1 extant.

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